Increasing automation and demands for quality mean that inspection by human beings becomes impractical in some situations. In these circumstances, it is not always necessary to image the internal structure of an object being inspected in order to detect an internal fault or extraneous body or other concealed object.
Existing inspection tools such as x-ray imaging systems employ an inspection medium (in the case of x-ray systems the inspection medium is a beam of x-rays). The inspection medium may in some cases be referred to as an illuminating beam or an inspection beam.
Beams of x-rays have been used to investigate the internal structure and properties of a range of objects including engineering materials and food products. However, x-rays and other forms of ionising radiation present a range of health and safety issues. Apparatus employing x-ray radiation must be heavily shielded to ensure that personnel are not exposed to radiation.
Moreover, the size of an x-ray system is large and not suitable for many inspection situations. A further disadvantage of x-ray devices is their relatively high cost.
Another disadvantage of x-ray systems is many materials such as very thin layers of wood, plastics and thin polymer films are relatively radiolucent and very difficult to detect via x-rays.
A fundamental physical limitation of x-ray systems when used in an inline inspection application is that they cannot as readily image/detect an foreign body or contaminant which is less dense than the medium in which it is embedded as compared to being able to detect an object which is much denser than the article in which it is embedded
Known inspection tools are typically imaging systems optimised for obtaining 2D or 3D spatially resolved images of a structure of a sample by exposing the sample to the inspection medium.
Image processing techniques are subsequently applied to the image to detect a presence of a foreign object, structural defect or other feature of interest. Such systems tend to be arranged to reduce or minimise scattering of the illuminating medium by the sample under inspection. This is in order to enhance a resolution of the imaging technique in some cases.
The present inventors have recognised that it is not necessary always to image an internal structure of a specimen to be inspected in order to conduct a meaningful inspection. Indeed, in some situations there may be an advantage in not imaging the internal structure. There may, in some situations, also be an advantage to enhancing the effects of internal scattering.
For example, it may be advantageous to employ an inspection technique where an object or feature to be detected interacts strongly with an inspection medium incident on the object or feature. The interaction may be characterised at least in part by distortion, scattering and/or absorption of the inspection medium.
The interaction may severely decrease the spatial resolution of an obtainable image of the object or feature. In many cases it may even render an image unobtainable However, if the principal aim of the inspection process is to determine whether or not the feature is present, and not to obtain a meaningful image of a structure of the object or feature, then such a technique may provide certain advantages.
For example, in the case of detection of an internal fault or extraneous object in a sample, it is desirable to be able to provide apparatus in which a detector can discriminate between a first condition in which a sample or portion of a sample not having an internal fault or extraneous object is present and a second condition in which a sample or portion thereof that does have such an internal fault or extraneous object is present. The greater the interaction of the inspection beam with the extraneous object, the greater the contrast at a detector between intensities of beams detected under the first and second conditions respectively.
Thus, it may be understood that a simple two-state detection system can be envisaged. The system may be provided in combination with a controller arranged to implement an algorithm arranged to provide a two state digital output, in accordance with Boolean principles. For example the controller may be arranged to output a high or low signal, or to illuminate a “red light” or “green light” depending on whether the first or second conditions described above are present.